As the data areal density in hard disk drives (HDD) continuously increases because of technology improvements, the magnetoresistive (MR) sensor that is used as the read-back element in HDD is required to have increasingly better spacial resolution while maintaining a reasonable signal-to-noise ratio (SNR). The sensor is a critical component in which different magnetic states are detected by passing a sense current through the sensor and monitoring a resistance change. A common giant magnetoresistive (GMR) configuration includes two ferromagnetic layers that are separated by a non-magnetic spacer in the sensor stack. One of the ferromagnetic layers is a pinned layer wherein the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) pinning layer. The second ferromagnetic layer is a free layer wherein the magnetization vector can rotate in response to external magnetic fields and is aligned either parallel or anti-parallel to the magnetization in the pinned layer. The spacer may be a conductive metal such as Cu in a giant magnetoresistive (GMR) device, or a dielectric layer in a tunneling magnetoresistive (TMR) sensor.
TMR sensors are commonly employed and provide a higher MR ratio (dR/R) than a GMR sensor where dR is the change in the resistance when a sense current is passed through the sensor, and R is the resistance of the sensor before the change. Advanced devices require a high MR ratio for improved signal-to-noise ratio (SNR) and a low RA product in order to reduce junction noise for better performance. However, it is quite difficult to simultaneously achieve an acceptable MR ratio and a RA product less than 1 ohm-μm2 because of the intrinsic physical limit of a MgO tunnel barrier layer. Typically, large interlayer coupling between the free layer and pinned layer, low MR ratio, and short lifetime due to pin holes in the tunnel barrier layer are the three main factors in hindering improvements in read head performance.
In order to meet the needs of current and next generation read head devices, it is necessary to reproducibly form a MR sensor with a MR ratio of 70% or higher and a RA product below 1 ohm-μm2. Although much effort has been applied to improving MgO barrier layer quality by depositing smoother films and by increasing film density, the pin hole issue continues to be an obstacle to realizing higher performance and good reliability. Therefore, a better tunnel barrier formation process is needed than currently available in the industry.